32 THE VELOCITY OF LIGHT. 



TELOCITY OF LIG11T EXPERIMENTAL DETERMINATION. 



lines. In using it, the two parts are placed at the distance 

 of several miles from each other, the separation being rep- 

 resented by the break in the dotted lines. The left-hand 

 part of the apparatus is simply a hollow tube, having at 

 the right-hand end of it a lens, and at the other end a mir- 

 ror, which are so adjusted that a beam of light entering 

 through the lens shall be brought to a focus on the mirror, 

 and then reflected back through the lens again on the same 

 path by which it came in. 



That an incoming and an outgoing radiation can thus 

 pass through the same tube, in contrary directions, at the 

 same time, without in the least degree interfering with or 

 deranging each other, is only another example of the won- 

 derful action of this mysterious power that was described 

 in the last chapter. 



The portion of the apparatus toward the right is set at 

 the place where the observation is to be made, the other 

 part, as has already been said, being placed at as great a 

 distance as possible, but within view. This second part of 

 the apparatus, like the other, consists of a tube, with a 

 branch near one end of it, which is open toward the source 



